1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery, more particularly to a non-aqueous electrolyte secondary battery which is most suitable for a thin type lithium-ion secondary battery, and the method of production thereof.
2. Description of the Related Art
Thin type lithium-ion secondary batteries have been commercialized as non-aqueous electrolyte secondary batteries for portable equipment such as cellular phones. This type of battery comprises a positive electrode made of lithium cobalt oxide (LiCoO.sub.2), a negative electrode made of graphitic and carbonaceous materials, an electrolyte solution prepared by dissolving a lithium salt in an organic solvent, and a separator made of a porous film.
Since the portable equipment is becoming increasingly thinner, it is a general requirement to reduce the thickness of batteries. However, it is difficult to produce thin type lithium-ion secondary batteries having a thickness of 4 mm or less. For this reason, card-type lithium secondary batteries having a polymer electrolyte have been proposed, and development for their commercialization has been in progress.
In lithium secondary batteries having a polymer electrolyte, a gel polymer holding a non-aqueous electrolyte solution is usually employed. Consequently, these batteries have larger impedance at the electrode interface and lower lithium-ion conductivity than non-aqueous electrolyte lithium secondary batteries. Further, when the positive- and negative electrodes of the lithium secondary batteries having a polymer electrolyte are made thinner in order to enhance lithium ion conductivity, the amount of the active material of the positive- and negative electrodes is reduced, thereby reducing the energy density.
Thus the lithium secondary batteries provided with a polymer electrolyte have a problem that they are inferior in volumetric energy density and large-current characteristics to thin type lithium secondary batteries which is impregnated with a non-aqueous electrolyte solution in a liquid state.
Japanese Patent Disclosure (Kokai) No. 10-177865 describes a lithium ion secondary battery which comprises a positive electrode, a negative electrode, a separator holding an electrolyte solution and having surfaces facing the positive electrode and the negative electrode, and adhesive resin layers disposed on the surfaces of the separator, which adhesive resin layers comprise a mixed phase of an electrolyte solution phase, a polymer gel phase containing an electrolyte solution and a solid polymer phase, said adhesive resin layers joining the positive- and negative electrodes with the adjacent surfaces of the separator. A claim of Japanese Patent Disclosure (Kokai) No. 10-189054 describes a method of producing a lithium ion secondary battery which comprises processes of forming positive- and negative electrodes on corresponding current collectors, coating a separator with a binder resin solution in which polyvinylidene fluoride is dissolved as a main component, forming a battery laminate by stacking the electrodes on the separator and drying the closely contacted electrodes and separator to vaporize the solvent, and impregnating the laminate with an electrolyte solution.
In these lithium ion secondary batteries, however, adhesive resin layers are interposed between the positive electrode and the separator and between the negative electrode and the separator. Consequently, these lithium ion secondary batteries have increased internal resistance, and inferior cycle life and large-current discharge characteristics.